Measure Exhaust port duration on PV motor?

DylanS

Gorilla Smasher
Location
Lebanon Pa
Can anyone help clear me up on how exactly to measure exhaust port duration on a PV motor (PV closed). I can’t find anything under search function. Seems like a dumb question but, unless my power valves are way to far from the piston, it seems exhaust gas could escape downward out underneath the valve before the piston is even level with it. Image below if that helps clarify. Am I supposed to measure from when edge of piston is level with the first bit of opening on the PV or as soon as the gas can get between the gap? Because if the last part is the case it seems almost useless to have the valve, unless my valves are way out of clearance. Thanks!
 

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Having the PV blade close to the piston is important.
Exhaust port measurements should be taken without the PV in mind.
The PV is simply a mechanical device used to hold the power charge in the cyl longer at lower rpm’s for more torque early in the rpm range, and they do make a noticeable difference. Cyl pressure increases with rpm forcing the valve open allowing more flow and HP higher in the rpm range.
It is important to be sure the valve is even with the roof of the exhaust port which you can verify by hand with the cyl open and the valve normally installed.
The entire PV theory is well matched for jet ski applications as a snappy torquey motor is what we need. Be happy you have them. They allow you to have higher exhaust port timing without sacrificing bottom end hit.


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DylanS

Gorilla Smasher
Location
Lebanon Pa
Having the PV blade close to the piston is important.
Exhaust port measurements should be taken without the PV in mind.
The PV is simply a mechanical device used to hold the power charge in the cyl longer at lower rpm’s for more torque early in the rpm range, and they do make a noticeable difference. Cyl pressure increases with rpm forcing the valve open allowing more flow and HP higher in the rpm range.
It is important to be sure the valve is even with the roof of the exhaust port which you can verify by hand with the cyl open and the valve normally installed.
The entire PV theory is well matched for jet ski applications as a snappy torquey motor is what we need. Be happy you have them. They allow you to have higher exhaust port timing without sacrificing bottom end hit.


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I know have a power valve works lol, I need to know how to properly measure my exhaust port timing with the power valve all the way down. For example the stock gp800 cyl opens the exhaust port at 78* and I’ve been told 90* with the PV shut. I want to know HOW someone got the measurement for the port timing with the PV closed. From what I gather they just had the piston level with the PV and as soon as the could see a gap between the two they finished their measurement. But what I’m saying is there is still easily enough clearance between the piston, valve, and exhaust port to have exhaust gas flow out BEFORE the piston is level with the valve (with the valve all the way down). When I said it “seems almost useless” I know that it’s impeding the flow of gases while it’s shut, but that gasses can technically flow out at a downward angle before the valve is even open in the slightest. That’s why I wanted to know what the clearance should be between PV to piston. I want to know what my port timing is with the valve shut because I’m trying to decide if I want to pin it at a set height on my 927 to fix the gp800 high exhaust port duration. Any chance you have inch specs between PV and piston? Thanks
 
I set mine up .015-.020 away from the piston. I have a 927 also and it took a lot of doing to make it happen. I have very high exhaust port timing also with the PV cyls and my motor rips from idle to WOT perfectly so you prob have something else going on. Never heard of anyone pinning a valve to reduce the effect of a high port


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I know have a power valve works lol, I need to know how to properly measure my exhaust port timing with the power valve all the way down. For example the stock gp800 cyl opens the exhaust port at 78* and I’ve been told 90* with the PV shut. I want to know HOW someone got the measurement for the port timing with the PV closed. From what I gather they just had the piston level with the PV and as soon as the could see a gap between the two they finished their measurement. But what I’m saying is there is still easily enough clearance between the piston, valve, and exhaust port to have exhaust gas flow out BEFORE the piston is level with the valve (with the valve all the way down). When I said it “seems almost useless” I know that it’s impeding the flow of gases while it’s shut, but that gasses can technically flow out at a downward angle before the valve is even open in the slightest. That’s why I wanted to know what the clearance should be between PV to piston. I want to know what my port timing is with the valve shut because I’m trying to decide if I want to pin it at a set height on my 927 to fix the gp800 high exhaust port duration. Any chance you have inch specs between PV and piston? Thanks
For low end timing. Timing is approximately when .005” shim/feeler gage fits between piston and PV closed. Difficult so I would just measure with a caliper from deck down to edge when closed and subtract squish. It’s not that big of deal IMO. I like the PV about .015-.020 minimum from wall. Did you look in the Yamaha manual on that spec?
 

DylanS

Gorilla Smasher
Location
Lebanon Pa
For low end timing. Timing is approximately when .005” shim/************ gage fits between piston and PV closed. Difficult so I would just measure with a caliper from deck down to edge when closed and subtract squish. It’s not that big of deal IMO. I like the PV about .015-.020 minimum from wall. Did you look in the Yamaha manual on that spec?
Ok, thanks that pretty much solved it for me. No I did not see it in the manual I don’t own one for this motor, I can dig around online tonight. I was using a .004 shim so that measurement helps too.
 
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DylanS

Gorilla Smasher
Location
Lebanon Pa
I set mine up .015-.020 away from the piston. I have a 927 also and it took a lot of doing to make it happen. I have very high exhaust port timing also with the PV cyls and my motor rips from idle to WOT perfectly so you prob have something else going on. Never heard of anyone pinning a valve to reduce the effect of a high port


Sent from my iPhone using Tapatalk
The motor ran good, but I wanted to measure everything out because the guy before me really ported the cylinders and I have a stroker crank in it. Just kind of make sure my numbers are in a decent ballpark. A buddy of mine had a stock bore gp800 that put our big bores to shame and I had read some thing about the exhaust port timing being thrown way off when they’re bored over.. thanks
 
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The motor ran good, but I wanted to measure everything out because the guy before me really ported the cylinders and I have a stroker crank in it. Just kind of make sure my numbers are in a decent ballpark. A buddy of mine had a stock bore gp800 that put our big bores to shame and I had read some thing about the exhaust port timing being thrown way off when they’re bored over.. thanks
When they are bored the timing gets lowered
 
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